Bolo 1872-S Seated Half Dollar Counterfeit

Any idea of which TPG slabbed them.

Quite possibly all of them. In the other cases in the past year or so when they have been found fakes in the genuine TPG slabs they have turned up at all four major companies.

As for the example pictured above, it is in a PCGS slab. (Look closely around the rims and you can see the three slab prongs.)

So what does it mean that the indicators both appear legitimately on 1875-S Seated Liberty coins? Why would the counterfeiter(s) use an 1875 coin/design/die to produce an 1872 counterfeit?

It was available.

That's how a LOT of these counterfeits get spotted. In order to cut expenses the counterfeiter will use one undated die with a whole series of dated ones. Back in the early years of these Chinese fakes it was possible to put together an entire date run of fake Half Cents (1793 - 1857) with the reverse die having been copied from an 1832 Half Cent rev. And it was done long befor the Chinese got involved. Think about the fake micro O Morgan dollars from the early 20th century. Three different years but they all use the same rev die.